Liquid ink printers of the type frequently referred to as continuous stream or as drop-on-demand, such as piezoelectric, acoustic, phase change wax-based or thermal, have at least one printing mechanism, such as a printhead, from which droplets of ink are directed towards a recording sheet. Within the printhead, the ink is contained in a plurality of channels. Power pulses cause the droplets of ink to be expelled as required from orifices or nozzles at the end of the channels.
In a thermal ink-jet printer, the power pulses are usually produced by resistors, each located in a respective one of the channels, which are individually addressable to heat and vaporize ink in the channels. As voltage is applied across a selected resistor, a vapor bubble grows in the associated channel and initially bulges from the channel orifice followed by collapse of the bubble. The ink within the channel then retracts and separates from the bulging ink thereby forming a droplet moving in a direction away from the channel orifice and towards the recording medium whereupon hitting the recording medium a dot or spot of ink is deposited. The channel is then refilled by capillary action, which in turn, draws ink from a supply container of liquid ink. Operation of a thermal ink-jet printer is described in, for example, U.S. Pat. No. 4,849,774.
The ink jet printhead may be incorporated into either a carriage type printer, a partial width array type printer, or a page-width type printer. The carriage type printer typically has a relatively small printhead containing the ink channels and nozzles. The printhead can be sealingly attached to a disposable ink supply cartridge and the combined printhead and cartridge assembly is attached to a carriage which is reciprocated to print one swath of information (equal to the length of a column of nozzles), at a time, on a stationary recording medium, such as paper or a transparency. After the swath is printed, the paper is stepped a distance equal to the height of the printed swath or a portion thereof, so that the next printed swath is contiguous or overlapping therewith. This procedure is repeated until the entire page is printed. In contrast, the page width printer includes a stationary printhead having a length sufficient to print across the width or length of a sheet of recording medium at a time. The recording medium is continually moved past the page width printhead in a direction substantially normal to the printhead length and at a constant or varying speed during the printing process. A page width ink-jet printer is described, for instance, in U.S. Pat. No. 5,192,959.
Printers typically print color and/or monochrome images received from an image output device such as a personal computer, a scanner, or a workstation. The color images are produced by printing with several colored inks or colorants of different colors at a time. The color of the ink and amount of ink deposited by the printer is determined according to image information received from a document creator such as a scanner or a computer workstation. The document creator provides an input digital signal gray-scale image, which is either defined in monochromatic terms, calorimetric terms, or both. The amount of gray level is typically defined by an input pixel value ranging from 0 to 255 where 0 is equal to white, 255 is equal to black, and values therebetween are shades of gray. For an image defined in colorimetric terms, the input pixel values are typically digital signals of R, G, B (red, green, and blue). Commonly this description may be part of a Page Description Language (PDL) file describing the document. In the case of computer generated images, colors defined by the user at a user interface of a workstation can be defined initially in a color space of tristimulus values. These colors are defined independently of any particular device, and accordingly reference is made to the information as being "device independent".
The printer, on the other hand, often has an output which can be defined as existing in a color space called CMYK (cyan-magenta-yellow-key or black) which is uniquely defined for the printer by its capabilities and colorants as well as the media upon which the printer deposits ink. Printers operate by the addition of overlapping multiple layers of ink or colorant in layers to a page or by the adjacent deposition of colorants. The response of the printer tends to be relatively non-linear. These colors are defined for a particular device, and accordingly reference is made to the information as being "device dependent". Thus, while a printer receives information in a device independent color space, the information must be converted to print in a device dependent color space, which reflects a possible range of colors of the printer, and secondly, printing of that image with a color printer in accordance with the colors defined by the scanner or computer generated image.
The perceived color of the image is determined not only by the relative amounts of each colorant put down on the recording medium, but also by the order in which the colorants are printed and the media type. Consequently, there are a multitude of variables which affects a final printed image. To accurately reproduce an original image, therefore, requires a transformation from a device independent color space to a multitude of device dependent color spaces, each one being determined, at least in part by, the combined effects of colorant, image density, media type, and print speed.
Various color printing systems and methods for printing color images on a recording medium are illustrated and described in the following disclosures which may be relevant to certain aspects of the present invention.
In U.S. Pat. No. 4,686,538 to Kouzato, a tone recording method wherein ink dots having at least two different concentrations per color are provided and the number of dots for each concentration to be printed in a pre-determined area is described. High and low density dots for yellow, magenta and cyan are described.
U.S. Pat. No. 5,161,010 to Birnbaum, describes an apparatus to determine the intensity of signals generated by a raster input scanner. Signals generated from a delay circuit and a gray detector are sent to a logic circuit which determines whether to output a gray signal or a non-gray signal to a raster output scanner.
U.S. Pat. No. 5,331,438 to Harrington, describes a method and apparatus for improving full color images using process black and printer black. An undercover removal and halftoning method which avoids abrupt density shifts and textures is described.
U.S. Pat. No. 5,631,748 to Harrington, describes halftone screens which are generated for each separation in accordance with a goal of avoiding overlapping whenever possible and with a goal of reducing interpixel contrast. Under color removal is selectively used to optimize the dot appearance. Black is increased or reduced in association with color increase or decrease to allow the appropriate space for filling with reduced luminance contrast.
The described invention applies to color systems having more than four colorants.